1. Field of the Invention
The present invention generally relates to current stabilization circuits, current stabilization methods, and solid-state imaging apparatuses, and particularly relates to a current stabilization circuit, a current stabilization method, and a solid-state imaging apparatus using such a current stabilization circuit wherein the current stabilization circuit can supply a stable electric current irrespective of the threshold voltage of a transistor, the power supply voltage, and operating temperature.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing an example of the construction of a related-art current amplification circuit. The current amplification circuit 10 of FIG. 1 includes transistors 11 through 15 and a resistor 16. The transistors 11 and the resistor 16 are connected in series, and a joint point between the transistor 11 and the resistor 16 is connected to both the gate of the transistor 11 and the gate of the transistor 12. With this provision, the transistors 11 and 12 constitute a current mirror circuit. Moreover, the transistor 12 and the transistor 13 are connected in series, and a joint point between the transistor 12 and the transistor 13 is connected to the gate of the transistor 13. In the example of FIG. 1, the transistors 12 and 13 have the same size as the transistor 11, so that a current i2 flowing through the transistors 12 and 13 is equal in amount to a current i1 running through the transistor 11.
The gate of the transistor 13 is connected to both the gate of the transistor 14 and the gate of the transistor 15, by which the transistors 13 through 15 constitute a current mirror circuit. In the example of FIG. 1, the transistor 14 is twice the size of the transistor 13 (twice the gate width), so that a current i3 flowing through the transistor 14 is twice as large as the current i2 flowing through the transistor 13. Further, the transistor 15 is four times the size of the transistor 13 (four times the gate width), so that a current i4 flowing through the transistor 15 is four times as large as the current i2 flowing through the transistor 13.
With this provision, the current amplification circuit of FIG. 1 generates the current i2 equal in amount to the base current i1, the current i3 twice as large as the base current i1, and the current i4 four times as large as the base current i1. The use of these currents i2 through i4 makes it possible to generate 8 different current levels corresponding to 3-bit values. By the same token, the generation of 8 electric currents being the same size, twice as large, four times as large, eight times as large, . . . , and hundred twenty eight times as large makes it possible to generate 256 different current levels corresponding to 8-bit values. The electric currents generated in such a manner may be used in a circuit portion for performing integration provided in the analog-to-digital converter of a solid-state imaging apparatus. In this integration circuit, a circuit as shown in FIG. 1 generates a desired amount of an electric current, and a capacitor is discharged with this desired current amount, thereby generating a ramp voltage having a desired slope (i.e., a voltage that decreases at a fixed rate). This ramp voltage is compared with a voltage read from the solid-state imaging device. A counter measures a time period that passes before the two voltages coincide, thereby converting the analog voltage into a digital value.    [Patent Document 1] Japanese Patent Application No. 11-161353    [Patent Document 2] Japanese Patent Application No. 2002-74997
In the circuit shown in FIG. 1, a change in the operating conditions and/or circuit conditions results in fluctuation of generated electric currents. If the power supply voltage fluctuates, for example, the voltage between the gate node and source node of each transistor changes, causing the current flowing through each transistor to fluctuate. If the threshold voltage of each transistor varies due to process variation, the current flowing through each transistor ends up varying in response to the threshold voltage. This happens even when the voltage between the gate node and source node of each transistor is maintained at a desired voltage. If there is a temperature change, further, the current flowing through each transistor changes because the drain current is related to the voltage between the gate and the source in a temperature-dependent manner.
Accordingly, a circuit as shown in FIG. 1 has difficulty supplying stable currents with sufficient accuracy when there is fluctuation in the operating conditions, circuit conditions, or the like. If the circuit shown in FIG. 1 is used in a solid-state imaging apparatus, the analog-to-digital converter will suffer a drop in conversion accuracy.
Accordingly, there is a need for a current stabilization circuit, a current stabilization method, and a solid-state imaging apparatus using such a current stabilization circuit wherein the current stabilization circuit can supply a desired current amount with sufficient stability and accuracy even if there is fluctuation in the power supply voltage, a change in the operating temperature, variation in the transistor threshold voltage, etc.